For my cryptography hobby (old school cryptograms, not modern cypto), I wrote this to find frequencies of letter pairs. I don't know if it could be adapted.

# 
# 
# my $input = shift;
# my $input = "peon bookkeeper";
my $input;
#print $input."\n";
while (<>) {
    $input = $_;
    @evenmatches = ($input =~ m/
                                (.{2}?)
                                (?{ #print $^N . " found at " . ($tmpp
+os = pos($input)) . "\n";
                                    $chars{$^N}++; 
                                  })
                               /xg);
    #print join(", ", @evenmatches)."\n";
    #print $#evenmatches + 1 ." matches found\n";
    
    #print $input."\n";
    pos($input) = 1;
    @oddmatches = ($input =~ m/
                                (.{2}?)
                                (?{ #print $^N . " found at " . ($tmpp
+os = pos($input)) . "\n";
                                    $chars{$^N}++; 
                                  })
                               /xg);
    #print join(", ", @oddmatches)."\n";
    #print $#oddmatches + 1 ." matches found\n";
}

print "frequency of '$_' is $chars{$_}\n"
    foreach (sort {$chars{$b} <=> $chars{$a}} keys %chars);
print "\n";    
print "frequency of '$_' is $chars{$_}\n"
    foreach (sort keys %chars);
[download]